There are many circumstances under which it is necessary to effect mixing of solids with liquids and/or solids under absolutely sanitary conditions. These include the production of food products, pharmaceutical products, mineral products and chemical products of high purity.
The products of the mixing can be in the form of mixed powders, wet granules, pastes or slurries.
In the production of the mixed products, it is essential that the products do not come into contact with contaminants, such as bearing lubricants. It is also essential that the mixing apparatus be easily cleaned to prevent contamination of the product with any built-up product which remains within the mixing apparatus. The apparatus should also be of a construction to preclude such build-up of product.
The mixing apparatus and method of the invention is directed to the mixing of solids with other solids and/or liquids and is distinguished from liquid-liquid mixers. In this regard, the mixing of solids with other solids and/or liquids is effected at high speeds of rotation, between 1000 and 4000 rpm, of a mixing shaft in a mixing chamber. This produces high shear forces and is contrasted with a liquid-liquid mixer in which the speed of rotation is much lower and relatively low shear forces are developed. Furthermore, because of vessel size, the shaft in the liquid-liquid mixers is able to deflect or tilt considerably without any adverse effect. In contrast, the mixing of solids with other solids and/or liquids is effected in a mixing chamber which has close tolerance with the shaft and permissable deflection or tilt of the shaft is minimal. Rotating shafts have a critical speed at which the shaft undergoes maximum deflection or tilt. The critical speed is a function of a number of factors, such as rigidity of the shaft, diameter and length of the shaft, type of support of the shaft etc. When the shaft of a mixer goes from rest to its operating speed during start-up and in reverse during stoppage, the shaft passes through its critical speed. In liquid-liquid mixers, this is of little significance and the critical speed may even be within the operating speed of the mixing shaft. In mixers of solids with other solids and/or liquids, the critical speed of the mixing shaft and the maximum deflection become important factors.
U.S. Pat. No. 3,887,166 discloses a mixer for mixing solids with liquids in which the wall of the mixing chamber is made of flexible material and the wall is constantly deformed during mixing to prevent build-up of deposits on the wall.
Although this mixer has proved to be of substantial value in continuously producing products under relatively controlled conditions, there is still a need for a mixer for producing highly pure products which meet stringent sanitary requirements. Namely, in this mixer unavoidable remnants of product are left behind in dead corners, in cracks and in crevices, which will contaminate the next load of product. Moreover, in the food, pharmaceutical and allied industries, there is the additional problem of bacteria growth at places which contain remnants of product. These two problems cannot sufficiently be solved by cleaning the mixer without disassembly.
Moreover, the conventional mixer is intended to operate continuously. This has the advantage over batch mixers in that it can produce large output more economically with less labor and the products are more uniform. However, if frequent changes are necessary for different product formulations, different dyes, different liquid binders and the like, the existing continuous mixer is disadvantageous as compared to batch mixers as the latter can be cleaned more quickly. Batch mixers are usually trough-shaped ribbon blenders or paddle mixers, conical mixers, fluid bed granulators, and the like. These are all easily accessible and cleanable. The more troublesome cleanability of the conventional mixers as disclosed in U.S. Pat. No. 3,887,166 causes extra down time between changeover of products.
In the food industry, the known mixers are used to transform powdery products into agglomerated products in order to instantize these products. Powdery food products, such as a cake or pudding mix, a sauce mix, etc. have a tendency to float on top of the liquids which are added to them, while subsequent stirring often leads to the forming of lumps. After stirring vigorously for a while, the total product, including some or most of the lumps, will disperse in the liquid (usually water or milk). In contrast, agglomerated products usually have good wettability, sinkability and dispersion speed in the liquids in which they have to be reconstituted. These characteristics make the agglomerated products instant.
In the pharmaceutical industry, the known mixers are used to transform powdered pharmaceutical compounds into free flowing masses prior to being fed into fast operating tableting machines. Powdery compounds have poor flow properties and cannot be fed quickly and evenly into the small molds of a tableting machine which compresses the product into a tablet. The increase in speed of current tableting machines necessitates even greater free flowing properties of the pharmaceutical compounds. Moreover, agglomerated products have better compressibility characteristics than powdery products.
In the chemical industry, special alloys in powdery form have to be agglomerated in order to end up with a free-flowing granular product which fills the molds quickly and completely in which special parts, such as tools, are fabricated by sintering. No contaminants should be present in the sintering stage.
In the case of pesticides, one plant should be able to produce, for example, broad-leaf and narrow-leaf herbicides. Obviously, cross contamination would create a herbicide which would destroy everything. Since there are formulations where only a few grams per acre of active matter are needed to obtain the desired effect, cross-contamination must be avoided.
The ceramic industry has applications where frequent color changes are necessary, for example, when producing all types of mosaic tiles. To ensure perfect color consistency of all batches of tiles, no matter when manufactured, it is imperative that there be no cross-contamination of dyes.
The above represent only a few of the multitude of applications where easy-cleaning and thorough cleaning of the agglomerator is of utmost importance.
In general, in order to effect mixing or agglomeration, the following conditions must be maintained.
No metal to metal connections without intermediate packing.
No cracks or dead corners.
Internal radii in corners.
Welded connections instead of bolted ones.
Easy assembly and disassembly for manual cleaning.